William Shatner returned to Star Trek to reprise the role of Captain James T. Kirk. The writers/directors guide, written, among others, by Gene Roddenberry and Jon Povill between May and August of 1977, described Kirk as followed:

"With the Starship out of communication with Earth and Starfleet bases for long periods of time, a Starship captain has unusual broad powers over both the lives and welfare of his crew, as well as over Earth people and activities encountered during these voyages. He also has broad power as an Earth Ambassador may discover. Kirk feels these responsibilities strongly and is fully capable of letting the worry and frustration lead him into error.

"He is also capable of fatigue and inclined to push himself beyond human limits, then condemn himself because he is not superhuman. The crew respects him, some almost to the point of adoration. At the same time, no senior officer aboard is fearful of using his own intelligence in questioning Kirk's orders and can themselves be strongly articulate up to the point where Kirk signifies his decision has been made.

"Kirk is a veteran of hundreds of planet landings and space emergencies. He has a broad and highly mature perspective on command, fellow crewmen, and even on alien life customs, however strange or repugnant they seem when reassessed against Earth standards.

"On the other hand, don't play Kirk like the captain of an 1812 frigate in which nothing or no one moves without his command. The Enterprise crew is a finely-trained team, well able to anticipate information and action Kirk needs.

"Aboard ship, Captain Kirk has only a few opportunities for anything approaching friendship. One exception is with ship's surgeon Dr. McCoy, who has a legitimate professional need to constantly be aware of the state of the Captain's mind and emotions. But on a "shore leave" away from the confines of self-imposed discipline, Jim Kirk is likely to play pretty hard, almost compulsively so. It is not impossible he will let this drag him at one time or another into an unwise romantic liaison which he will have great difficulty disentangling. He is, in short, a strong man forced by the requirements of his ship and career into the often lonely role of command, even lonelier because Starship command is the most difficult and demanding task of his century."

Elements of the character of Xon, such as his search to understand humans, would be transferred later into TNG's character of Data. Also the concept of a full-blooded Vulcan having to deal with 'barbaric' humans is explored with T'Pol aboard the Enterprise NX-01.

The writers/directors guide, written, among others, by Gene Roddenberry and Jon Povill between May and August of 1977, described Xon as followed:

"Can a twenty-two-year-old Vulcan on his first space voyage fill the shoes of the legendary Mr. Spock? Xon (pronounced Zahn) was selected by the Vulcan Science Academy to attempt exactly that. Kirk was stunned when his new science officer reported aboard and found him to be a little more than a boy. (Xon looks something like a young Michael York with pointed ears.) Kirk has assumed the replacement was someone near Spock's age. The reports he had read on Xon listed him as a prominent scientist and teacher.

"The truth is that Xon is a genius, even by Vulcan standards. As we'll see in our episodes, he is as competent as Spock in all fields of science. He lacks knowledge, however, in one very important area – the Human equation. Unlike Spock, Xon is a full Vulcan. He had no Human mother to acquaint him with the Earth species; he has no Human half with which to feel and understand Human emotions.

"Xon realizes that the reason that Spock performed so well in his tasks on board the Enterprise was that he was half Human and therefore could understand emotional Human nature. In order to perform as well as Spock, he knows he is going to have to eliminate his Vulcan revulsion at emotional displays. He is, in fact, going to have to reach down within himself and find the emotions that his society has repressed for thousands of years so that he will have some basis for fully understanding his Human associates.

"What this means is: whereas Spock was engaged in a constant battle with himself to repress his emotions in order to be more Vulcan-like, Xon will be engaged in a constant struggle within himself to release his buried emotions to be more Human-like for the sake of doing a good job, his primary considerations. This will be at least as difficult for him as it was for Spock to maintain his stoic pose. Also, we'll get humor out of Xon trying to simulate laughter, anger, fear, and other Human feelings.

"The new science officer accepted the Enterprise assignments with much trepidation. He has no doubt that he can competently handle the scientific aspects of his job, but he fears the crew might expect him to be a duplicate of Spock as well as a replacement. These fears have been realized and hanging over the early episodes. So also is the unsaid comment, "Mr. Spock never did it quite like that." Nor is Captain Kirk overly fair to Xon in the beginning. Spock's friendship was a deep, important thing to Kirk, and the Captain is now almost arbitrarily rejecting the possibility of a meaningful relationship with the young Vulcan. However, the more difficult Lieutenant Xon's situation, the more we'll like him and the more we'll want him to succeed in this difficult assignment.

"As a full Vulcan, Xon is even stronger than Spock. He can endure lack of water and high temperatures for very long periods. All his senses are particularly keen. He has strong Vulcan mind-meld abilities.

"The young Vulcan lieutenant is constantly shocked by Human behavior. In preparing for this assignment, he made himself quite an expert on Human behavior and history. And it is amusing to see him try to apply this knowledge too logically and too literally. Nothing he studied quite prepared him for the real thing. Although Xon tries hard to hide his surprise and discomfitures, the crew is aware that it exists. They often go out of their way to exaggerate their Human qualities, further distressing the young Vulcan. But this is not done in mean spirit and never in a situation where it will interfere with starship efficiency. We will suspect that life among Humans is causing Xon to begin to feel some emotions himself. On his planet this is, of course, grossest of sins and the young Vulcan makes every effort to hide any sign of this "weakness."

"The science officer presides over a large console which is known as the "Library Computer Station." It is second in importance only to ship command and is located directly behind Captain's position."

Willard "Will" Decker was another of the new characters among the cast, supposed to fulfill the role of ship's first officer. The role had not been cast at the time of the series shutdown; Stephen Collins played the character on The Motion Picture. Much of what had been laid down for the character of Decker formed the basis for First Officer William "Will" Riker of The Next Generation, as is apparent from how the character is described in the writers/directors guide, written, among others, by Gene Roddenberry and Jon Povill between May and August of 1977.

"In his youthful thirties, Decker is the ship's Executive Officer, second in command. Kirk sometimes calls or refers to him as "First", which is naval parlance for ship's "First Lieutenant," which would have been Decker's title in the days of sailing ships. Will Decker comes very near to worshiping Kirk and would literally rather die than fail him. The prime responsibility of a "First" is to provide his captain with the most efficient crew and vessel possible and Will Decker takes this responsibility seriously.

"When not absorbed in his task of keeping the Enterprise at top fitness, Will Decker is a very humorous man. He particularly enjoys playing the "too perfect," soulless marionette of an officer. The joke can be confusing to others because Will can almost become that kind of officer when Kirk's welfare or the strategy of the ship is involved.

"We can see that Jim Kirk is very much in the process of training the young commander for the responsibilities of Starship command someday. We will see that future captain begin to happen during this five-year mission.

"In areas of logistics and organization, he has a keen and analytical mind, one upon which Kirk will rely heavily. He will command some landing parties and many decisions will be life-and-death choices.

"Will's background is all service: his father, his father's father were Academy graduates, Starfleet officers of flag rank. Someday, surely, he will wear a star. Because of his heritage, and because he has been groomed since nearly birth for command. He has friends, but tends to protect his privacy while respecting others'. Between Kirk and Decker is a kind of father/son relationship that each cherishes."

The third new character to Star Trek came in the form of Lieutenant Ilia (pronounced "Il-ee-ah"), performed by Persis Khambatta. Gene Roddenberry's and Jon Povill's 1977 writers/directors guide described her as:

"...a young female of Planet 114-Delta V, which has recently joined the Federation. The Deltan race is much older than Humans, with brains much more finely evolved in areas of art and mathematics. These abilities make her a superb navigator and her artistic abilities are evident in her sure, flowing precision at this task.

"Her face is breathtakingly beautiful. But like all Deltans, she is completely hairless except for the eyes. Her smooth, slender bare head has the almost sensually quality of delicately contoured nudity, always hidden before in other women. It gives her a striking, almost "Egyptian" look, particularly when wearing a Deltan jewel-band head ornament.

"Ilia's intelligence level is second only to the Science Officer, and she has also the esper abilities common on her planet. Unlike the mind-meld of Vulcans, it simply is the ability to sense images in other minds. Never words or emotions, only images... shapes, sizes, textures. On her planet, sexual foreplay consists largely of lovers placing images in each other's minds.

"Just as Vulcans have a problem with emotions, Ilia has a problem which accompanies her aboard the starship. On 114-Delta V, almost everything in life is sex-oriented; it is a part of every friendship, every social engagement, every profession. It is simply the normal way to relate with others there. Since constant sex is not the pattern of humans and others board this starship, Ilia has totally repressed this emotion drive and social pattern."

Certain characteristics of Lieutenant Ilia, such as her relationship to the young first officer and empathic abilities later formed the basis for the character of Counselor Deanna Troi of The Next Generation.

"Senior Ship's Surgeon of the USS Enterprise, head of the Medical Department. As such, he is responsible for the health and physical welfare of the crew of the Enterprise. He also has broad medical science responsibilities in areas of space exploration.

"As Senior Ship's Surgeon, "Bones" McCoy is the one man who can approach Captain Kirk on the most intimate personal levels relating to the Captain's physical, mental and emotional well being. Indeed, he has the absolute duty to constantly keep abreast of the Captain's condition and speak out openly to Kirk on this matter. McCoy is portrayed as something of a future-day H.L. Mencken, a very, very outspoken character, with more than a little cynical bite in his attitudes and observations of life. He has an acid wit which results in sometimes shocking statements–statements which, under close scrutiny, carry more than a grain of truth about medicine, man and society.

"Of all the men aboard our starship, McCoy is the least military. He is filled with idiosyncrasies which fit the character and are his trademark. For example, he loathes the transporter and system of "beaming" personnel from the ship to planet surfaces, and loudly proclaims that he does not care to have his molecules scrambled and beamed around as if he were a radio message.

"McCoy is highly practical in the old "general practitioner" sense, hates pills except when they are vitally needed, is not above believing that a little suffering is good for the soul and the maturity of the individual. He has a great fear that perfect medicine, psychotherapy and computers may rob humankind of his individuality and his divine right to wrestle a bit with life. He's a superb physician and surgeon–often seems to be treating the wrong ailment–but usually is proven right in the end.

"Dr. McCoy is the oldest crew member aboard, and as such, subject to some ribbing. He was married once, something of a mystery that ended unhappily. He is a grandfather, but unhappily his starship duty has prevented him forming the relationship with his grandchildren he would have desired. His years provide him wisdom and experience, and offer an interesting – and sometimes poignant – counterpoint to the younger officers and crewmen.

"Lieutenant Xon, like Spock before him, appears to regard McCoy as a bumbling country doctor, generally achieving cures through luck rather than science. But "Bones" McCoy, like most cynics, is a at heart a bleeding Humanist and the affectionate (and humorous) feud that was carried on between Spock and McCoy is continued between McCoy and Xon.

"With the considerable difference, however, that McCoy feels the "feud" is a very private affair concerning himself and Xon – and McCoy has been known to severely chastise (in private) those crewmen and officers who have been guilty of unfairness to the young Vulcan in comparing his efforts to Spock's. If you accuse McCoy of protecting Xon, he would vehemently deny it."

James Doohan was to reprise his role of Chief Engineer "Scotty" on Phase II. Gene Roddenberry's and Jon Povill's 1977 writers/directors guide described him as having:

"...a rare mechanical capability many claim he can put an engine together with baling wire and glue... and make it run. He regards the USS Enterprise as his personal property and the Engineering Section as his private world where even Captain James Kirk is merely a privileged trespasser.

"Engineering and spaceships are his life. His idea of a pleasant afternoon is tinkering in an Engineering Section of the vessel; he is totally unable to understand why any sane man would spend reading time on anything but technical manuals. He is strong-minded, strong-willed and not incapable of telling off even a Starfleet Captain who intrudes into what Scotty regards as his own private province and area of responsibilities.

"Kirk understands his Engineering Officer's fierce love of his vessel and his engines, will take more "guff" off this officer than almost any other aboard the ship. Regarding him, Kirk has one rule: "If it doesn't run, take it to Scotty. If he can't fix it, it's irreparable.""

"Rank of Lieutenant Commander, Communications Officer, played by attractive young actress Nichelle Nichols. Uhura was born in the African Confederacy. Quick and intelligent, she is a highly efficient officer. Her understanding of the ship's computer systems is second only to the Vulcan Science Officer, and expert in all ships systems relating to communications. Uhura is also a warm, highly feminine female off duty. She is a favorite in the Recreation Room during off duty hours, too, because she sings – old ballads as well as the newer space ballads – and she can do impersonations at the drop of a communicator."

"Ship's helmsman, played by actor George Takei. Mixed Oriental in ancestry, a Lieutenant Commander, Japanese predominating, Sulu is very Occidental in speech and manner. In fact, his attitude toward Asians is that they seem to him rather "inscrutable." Sulu fancies himself more of an old-world "D'Artagnan" than anything else. He is a compulsive hobbyist; like all "collectors," he is forever giving his friends a thousand reasons why they, too, should take on the same hobby.

"Although these bursts of enthusiasm make him something of a chatterbox, Sulu is a top officer and one of the most proficient helmsmen in the Starfleet Service. When the chips are down, he immediately becomes another character, a terse professional, whose every word and deed relate solely to the vessel and its safety. This pleasant and effective "dual personality" results never intrude on his job. He has never had to receive the same order from Kirk twice."

Walter Koenig was to reprise his role of Pavel Chekov on Phase II. Gene Roddenberry's and Jon Povill's 1977 writers/directors guide described him as follows:

"Formerly an ensign, the youngest officer aboard, Chekov is now a full lieutenant with years of space adventure behind him. He commands the security division of the USS Enterprise, and is responsible for matters of security and discipline both aboard the vessel and ashore. He is responsible also for the training of the men and women who make up his security teams. During action stations, his post is on the bridge at the damage control console. The Captain's safety is Lt. Chekov's responsibility, too, very much as the Captain's health is McCoy's concern."

Majel Barrett was to reprise her role of Christine Chapel on Phase II. Originally a nurse on the original Star Trek, she returned as a full doctor to serve as McCoy's associate. Gene Roddenberry's and Jon Povill's 1977 writers/directors guide described her as being:

"...second in command of the ship's medical section, and McCoy seems to enjoy passing on to her every duty he finds too boring, irritating or annoying to do himself. Yet outside of Captain Kirk, she is probably McCoy's closest confidante. An expert in psychotherapy, she has unusual ability to teach patients how to use the healing power of their own bodies."

"Played by a succession of young actresses, always lovely. One such character has been well-established, "Yeoman Janice Rand," played by the lovely Grace Lee Whitney. It is a tradition of Starfleet that yeomen are invariably female and serve ship commanders as combination of executive secretary-valet-military aide. It is a much sought-after post because of the experience gained and many yeomen go on to eventually become senior bridge officers and Starfleet captains. As in the case of all females aboard, they are treated co-equally with males of the same rank and the same level of efficient performance is expected. The yeoman often carries an over-the-shoulder case, the tricorder, which is an electronic recorder-camera-sensor combination, immediately available to the captain, should he be away from his command console."